WO1995023297A1

WO1995023297A1 - Automatic position-sensing active magnetic bearing

Info

Publication number: WO1995023297A1
Application number: PCT/FR1995/000231
Authority: WO
Inventors: Ulrich Schroeder; Maurice Brunet
Original assignee: Societe De Mecanique Magnetique
Priority date: 1994-02-28
Filing date: 1995-02-28
Publication date: 1995-08-31
Also published as: DE69503613D1; FR2716700A1; US5844339A; JP3382947B2; JPH09510280A; FR2716700B1; DE69503613T2; EP0749538A1; EP0749538B1

Abstract

An automatic position-sensing active magnetic bearing comprises first and second opposing electromagnets arranged on either side of a ferromagnetic body (1) suspended without contact between said electromagnets. The bearing comprises inductive-type position sensors formed by the coils (11; 21) of the first and second electromagnets. Means (14; 24) are provided to inject simultaneously at the input of the power amplifiers (13; 23) supplying the windings (11; 21) of the first and second opposing electromagnets, in superposition to the main current from the control circuits (31, 32), a sinusoidal current Io sin wt having a constant amplitude Io of w pulsation and an identical phase. Circuits (41 to 44) derive position information determining the intensity of the main current to be applied by the control circuits (31, 32) to the power amplifiers (13, 23) directly from voltages u1, u2 at the terminals of the excitation coils (11; 21), said voltages being measured according to the frequency of the sinusoidal current forming a w pulsation carrier.

Description

Active magnetic bearing with position auto-detection

The present invention relates to an active magnetic bearing with position auto-detection comprising first and second opposing electromagnets arranged on either side of a ferromagnetic body suspended without contact between these electromagnets, the first and second electromagnets oriented along an axis X * X each comprising on the one hand a magnetic circuit defining with said body an air gap ej; e2 whose average value is equal to a predetermined common value e ₀ , and on the other hand an excitation coil supplied from a power amplifier whose input current is controlled as a function of the position of the body by compared to the magnetic circuits of the first and second electromagnets.

In the most commonly used active magnetic bearing systems, the position of the suspended body, which body can be, for example, a machine rotor or a disc, is constantly monitored using position detectors associated with the electromagnets of the activator bearings producing the forces necessary to keep the body suspended in a given working position.

The fact of using position detectors distinct from the electromagnets supporting the suspended body considerably increases the cost of the machines, because of the detectors themselves, of the associated connectors and of the electronic circuits for supplying them.

In addition, these detectors can only be located next to the landing, and therefore are not collinear as a detection point relative to the reaction point. This relocation of the detectors relative to the actuating members can induce perverse effects as regards the control of the rotor, when it is a question of taking into account the deformations of the rotor induced during rotation. The detectors may in fact then be at deformation nodes while the activating electromagnets will be closer to deformation bellies, or vice versa. The information provided by the detectors will then not be a true reflection of reality at the level of the actual stops.

Also known from document WO-A-93/23683 is a self-sensing magnetic bearing system comprising, for a given direction, coils connected in series in which both a current intended to stabilize the position of the mobile body suspended by the bearing and a detection current. Such a magnetic bearing system however requires a stator pre-magnetization carried out with permanent magnets as well as amplifiers capable of supplying each of the currents which can be in turn positive and negative depending on the position of the mobile body. The use of pre-magnetization means and complex linear amplifiers of the four-quadrant type makes the implementation complex and affects the reliability of the assembly.

The present invention aims to remedy the various aforementioned drawbacks and to ensure reliable control of the electromagnets of an active magnetic bearing by providing inexpensive position information of the suspended member without using detectors position sensors, such as inductive, capacitive or optical detectors. The invention aims more particularly to make it possible, with conventional active magnetic bearings using, in a given direction, only two electromagnets in opposition producing forces of attraction on a mobile body, a position detection of the inductive type without the need to add to the magnetic bearing separate detection coils of the bearing electromagnets themselves, or pre-magnetization means.

The invention aims to allow the realization of active magnetic bearings of relatively low cost, in particular for machines to be produced in large quantities, such as for example textile spindles, machining spindles, vacuum pumps, discs computers.

These objects are achieved, in accordance with the invention, thanks to an active magnetic bearing with position auto-detection comprising first and second antagonistic electromagnets arranged on either side of a ferromagnetic body suspended without contact between these electro -magnets, the first and second electromagnets oriented along an axis X'X each comprising on the one hand a magnetic circuit defining with said body an air gap whose average value is equal to a predetermined common value e ₀ , and on the other share an excitation coil supplied from a power amplifier whose input current is controlled as a function of the position of the body with respect to the magnetic circuits of the first and second electromagnets, characterized in that the power amplifiers are constituted by switching amplifiers; in that the coils of the first and second electromagnets directly constitute position detectors of the inductive type; in that means are provided for simultaneously injecting at the input of the power amplifiers supplying the coils of the first and second opposing electromagnets, superimposed on the main current coming from the servo circuits, a sinusoidal current _j sin wt d ' constant amplitude, of pulsation w and having an identical phase, the ratio between the detection voltage supplied by an amplifier at pulsation w and the voltage intended to supply the force exerted by the corresponding electromagnet being of the order of 5 at 20% and preferably close to 10%; and in that it comprises circuits for extracting position information determining the intensity of the main current to be applied by the servo circuits to the power amplifiers directly from the voltages uj, U2 at the terminals of the excitation coils , measured at the frequency of the sinusoidal current constituting a pulsation carrier w

Thus, in accordance with the invention, the magnetic bearing, in addition to its actual bearing function, is also used as a position detector of the inductive type, without it being necessary to add any additional coil, the voltage being measured at terminals of each solenoid coil of the bearing in conditions for which there is almost proportionality with the movement of the suspended body (rotor).

The fact that the sinusoidal current of constant amplitude I ₀ and fixed pulsation w is injected simultaneously into the two opposing electromagnets, with an identical phase on both sides, avoids creating the least disturbing force on the rotor when it this is in its central nominal position, in particular in cases where the frequency of the sinusoidal current constituting the carrier must be low enough to limit the voltage consumption supplied by the power amplifier supplying the bearing.

In all cases, the active magnetic bearing is characterized in that the frequency of the sinusoidal current injected with pulsation w is significantly greater than the desired bandwidth in closed loop of the servo circuits. The use of switching power amplifiers makes it possible to limit losses and to supply the bearing coils with a very high reactive power.

Advantageously, the switching amplifiers have a switching frequency of the order of several tens of kilohertz.

According to an important characteristic of the present invention, the voltages uj, U2 at the terminals of the excitation coils measured at the frequency of the sinusoidal pulsating current w are compared in differential measurement in a comparator providing a voltage u proportional to the displacement of the body relative to its equilibrium position.

The differential measurement eliminates the two constant terms of the form U ₀ = LI ₀ W, due to the inductance L _Q of the electromagnet coils relative to the mean air gap e ₀ , and the constant terms U _s I ₀ w due to the leakage inductance Lς of the electromagnet coils relating to magnetic leaks.

More particularly, the position information extraction circuits include a bandpass filter centered on the frequency of the sinusoidal pulsating current w and having a half-width adapted to the desired bandwidth of the servo circuits. The position information extraction circuits include a synchronous demodulator placed at the output of the bandpass filter and controlled by the frequency of the sinusoidal pulsating current w.

The position information extraction circuits include a second order filter for filtering the frequency corresponding to a 2w pulse created by full wave demodulation within the synchronous demodulator.

Advantageously, the switching frequencies of the switching power amplifiers and the frequency of the sinusoidal pulsating current w are synchronized. This avoids noise problems due to frequency beats.

The electromagnets are dimensioned in such a way that the maximum force required from these electromagnets is obtained for a magnetic induction situated outside the saturation zone of the material used to constitute the magnetic circuits of the electromagnets. Since the path in the magnetic circuits of the bearing electromagnets remains negligible and remains almost constant whatever the force required of the electromagnets, the measurement of the inductance of the coils of the bearing electromagnets may well remain inversely proportional to the air gap.

The magnetic circuits of the electromagnets are made of a material with little variable permeability as a function of magnetic induction, in particular in the operating zone of the electromagnets located far from the saturation zones. The servo circuits supplying the power amplifiers comprise a comparator between the position signal coming from the position information extraction circuits and a position reference value, and at least circuits comprising a correction processing network for signal and a linearizer of the quiescent current. The servo circuits can also comprise flow regulation loops interposed between on the one hand the circuits comprising a corrector network and a linearizer, and on the other hand adder circuits constituting said injection means receiving said sinusoidal current I ₀ sin wt of pulsation w. In this case, by way of example, each flow regulation loop comprises a rejector filter centered on a pulse frequency w and receiving as input the output voltage signal from the corresponding amplifier, a corresponding flow calculation circuit , a flow calculation circuit to which are applied on the one hand the output signal of the corresponding rejector filter and on the other hand the output signal of a flux regulator also applied to an input of the corresponding adder circuit and a subtractor circuit which subtracts from the signal from circuits comprising a correction network and a linearizer, the signal from the corresponding flux calculation circuit and delivers an output signal to the input of the corresponding flux regulator circuit.

Naturally, the invention covers active magnetic bearings comprising in particular two active magnetic bearing systems of the type defined above oriented in two directions XX ', YY' perpendicular to each other. Other characteristics and advantages of the invention will emerge from the following description of particular embodiments of the invention, given by way of nonlimiting examples, with reference to the appended drawing in which: - Figure 1 is a diagram block of an active magnetic bearing system according to the invention incorporating a position detection system using the bearing electromagnets, and

- Figure 2 is a curve showing the shape of the position signal delivered by the position detection system, as a function of variations in the air gap of the electromagnets.

FIG. 1 shows a rotor 1 suspended by two opposing bearing electromagnets oriented along an axis XX 'perpendicular to the axis of the rotor 1. Each electromagnet comprises a magnetic circuit 12 resp. 22 made of ferromagnetic material having in cross section the shape of a U with two pole pieces 121, 122 resp 221, 222 oriented towards the rotor 1, itself made of ferromagnetic material or coated with a frame of ferromagnetic material. The pole pieces 121, 122 each defining with the rotor 1 an air gap c \ = e ₀ + x where e ₀ represents the mean air gap when the rotor is in its equilibrium position and x represents the variation of the air gap with respect to at the mean value e _Q , generated by a displacement of the rotor 1 in the radial direction X'X. The pole pieces 221 and 222 are symmetrical with the pole pieces 121, 122 with respect to the axis of the rotor 1 when the rotor 1 is in its equilibrium position, so that the pole pieces 221, 222 each define with the rotor 1 an air gap e2 = e ₀ - x where e ₀ represents the average air gap when the rotor 1 is in its equilibrium position and x represents the variation of the air gap compared to the average value e ₀ generated by a displacement of the rotor 1 in the radial direction X'X. Thus, a positive value of x will mean a displacement of the rotor 1 towards the magnetic circuit 22 while a negative value of x will mean a displacement of the rotor 1 towards the magnetic circuit 12.

Each electromagnet further comprises an excitation coil 11 resp 21 which is supplied from a power amplifier 13 resp 23, which is preferably an amplifier with switching in current mode. Each power amplifier 13 resp 23 receives an input main current which makes it possible to supply the coils 11, 21 the energy required to maintain the rotor 1 in its predetermined equilibrium position. The main input current comes from servo circuits having themselves received information on the actual position of the rotor and taking into account a setpoint position. Each amplifier 13 resp 23 delivers a unidirectional current independent of the current delivered by the other amplifier 23 resp 13, the electromagnets supplied independently by the amplifiers 13, 23 being mounted in opposition and producing forces of attraction on the movable body 1 on either side thereof along the axis X'X. To allow position information to be obtained without using a position detector separate from the bearing electromagnets, adder circuits 14, 24 make it possible to simultaneously inject power amplifiers 13, 23 at the input, superimposed on the current main from the servo circuits, a sinusoidal current of the form I _Q sin wt of constant amplitude I ₀ , of pulsation w and having an identical phase.

At the output of the power amplifiers 13, 23, it is therefore injected, superimposed on the current intended to supply the bearing force of the electromagnets, currents ij, v of the form: = i2 = Io sin wt. The air gaps ej and e2 between the rotor 1 and the poles 121, 122 resp 221,

222 are of the form indicated above, namely: ei = e ₀ + xe ₂ = e ₀ - x

The inductances of the electromagnets 11, 12 resp 21, 22 are respectively of the form:

Li ≈ Ls + Lo / Cl + x eo)

L ₂ = Lg + Lo / (l "x / e ₀ ) where L ^ represents the leakage inductance of the electromagnets 11, 12 resp 21, 22 relating to magnetic leaks, and L _Q represents the inductance of the electro- magnets 11, 12 resp 21, 22 relating to the mean air gap e ₀ .

The voltages 41 and 42 at the input terminals of the coils 11 and 21, due to the injected currents \\ and i2, are of the form: uj = - w I _Q L COS wt U2 = - w I ₀ L2 cos wt

SUBSTITUTE SHEET {RULE 26) When one carries out in a comparator 41 the difference of the tensions uj and U2, one obtains expressions of the form: u = uj - U2 = - j cos wt (Lj - L2)

= (wL _j cos wt) .2 L _Q. (x / e ₀ ) / (1 - (x / e ₀ ) 2) = Af (x / e ₀ ) where A = (wL _j cos wt) 2 L _Q = constant and f (xe ₀ ) = (x / eo) / (l - (xyeo) ² )

We see that the comparison in differential measurement of the voltages u \ and U2 makes it possible to eliminate the constant terms due to the inductance L _Q relative to the mean air gap e ₀ and to the leakage inductance Ls relative to the magnetic leaks so that the resulting voltage is proportional to the displacement x of the rotor 1.

The voltage u is filtered in a bandpass filter 42 centered on a frequency corresponding to the pulsation w, and having a half-width adapted to the desired passband of the servo-control of the magnetic bearing system.

The voltage u is then demodulated in a synchronous demodulator 43 controlled by a frequency corresponding to the pulsation w.

The output of the synchronous demodulator 43 provides, after passing through a second order filter 44 ensuring the filtering of the frequency created by the half-wave demodulation, a signal representative of the actual position of the rotor along the axis X'X, at the level of the bearing electromagnets.

This signal is of the form: u (x) = k (x / e ₀ ) / (l - (x / e ₀ ) 2) Figure 2 shows the linearity of the position signal u (x) obtained, depending on the ratio x / e ₀ between the displacement of the rotor x and the average air gap e ₀ -

It can be seen that this linearity is very good for the usual working range of the bearing. It is thus possible to deduce a rotor position signal from the measurement of the inductance of two opposing bearing electromagnets, without adding a separate detector to the bearing electromagnets and without creating the least disturbing force on the rotor. when the latter is in its nominal central position, as soon as the injected current of the form I ₀ sin wt is injected simultaneously into the two opposing electromagnets, with an identical phase on both sides, which allows in particular a comparison in differential measurement.

The variable quiescent currents injected into the amplifiers represent a low value compared to the servo currents intended to supply the force exerted by the electromagnets, and the corresponding rotor losses can remain extremely low.

The frequency of the sinusoidal current injected, corresponding to the pulsation w, is significantly higher than the closed-loop bandwidth of the system, but should preferably remain low enough to limit the voltage consumption supplied by the power amplifiers supplying the level.

As already indicated above, the amplifiers 13, 23 are advantageously constituted by switching amplifiers, of a type known per se, to limit losses. These switching amplifiers may have a switching frequency which may be of the order of, for example, 50 kHz or even be greater than this value and may equal or exceed 100 kHz. However, it is preferable to synchronize the switching frequencies of the amplifiers 13, 23 and the frequency of the pulsed injected carrier w in order to avoid the problems of noise due to frequency beats.

By way of example, the switching frequency may be of the order of twenty times the detection frequency corresponding to the pulsation w, and the detection frequency may itself be of the order of twenty times the frequency of enslavement. In general, the bearing is dimensioned so that the maximum force requested from the bearing is obtained for an induction situated outside the saturation zone of the material used for the bearing, and the magnetic circuits 12, 22 of the bearing are produced made of material with little variable permeability as a function of magnetic induction, in particular in the operating zone of the electromagnet, that is to say of its saturation zone.

It will be noted that the active magnetic bearing system with auto¬ detection does not involve any pre-magnetization by permanent magnets, which simplifies the production and increases the robustness and the reliability. The position signal from filter 44 can then be used in servo circuits in the traditional way, as if this signal had been obtained by an independent detector, but with the guarantee that the information corresponds exactly to the location of the electro- bearing magnets.

The position signal can thus be compared in a comparator 31 with a position reference value and the signal from the comparator 31 is applied to circuits 32 which may include a signal processing correction network, for example of the PID type (proportional- Integral- Derivative) and a quiescent current linearizer. The signal from circuits 32 can then be applied, with different signs, to one of the inputs of each of the adder circuits 14, 24 placed at the input of amplifiers 13, 23.

Advantageously, the servo circuits can further comprise flow regulation loops interposed between on the one hand the circuits 32 and on the other hand the adder circuits 14, 24.

Each flow regulation loop can include a rejector filter 15; 25 centered on a pulse frequency w and receiving as input the output voltage signal from the corresponding amplifier 13; 23, a flow calculation circuit 16; 26 to which are applied on the one hand the output signal of the corresponding rejector filter 15; 25 and on the other hand the output signal from a flow regulator 18; 28 also applied to an input of the corresponding adder circuit 14; 24 and a subtractor circuit 17; 27 which subtracts from the signal from circuits 32, the signal from the corresponding flux calculation circuit 16; 26 and delivers an output signal to the input of the corresponding flow regulator circuit 18; 28.

The use of a flux loop by exploiting the output voltages of the amplifiers, that is to say the signal also used for detection, ensures better dynamic behavior, even with a bandwidth of the position detection system. scaled down.

Naturally, it is possible to incorporate, for example in a radial active magnetic bearing, two active magnetic bearing systems according to the invention each comprising a pair of antagonistic electromagnets, the two pairs of electromagnets being oriented according to two directions XX ', YY' perpendicular to each other. The active magnetic bearing according to the invention can also be used alone, for example as an axial bearing cooperating with the two faces of a disc. In this case, the electromagnets undergo a rotation of 90 * with respect to the case of FIG. 1, the pole pieces 121, 122, 221, 222 becoming parallel to the axis of the disc.

Claims

1. Active magnetic bearing with position auto-detection comprising first and second opposing electromagnets arranged on either side of a ferromagnetic body (1) suspended without contact between these electromagnets, the first and second electro- magnets oriented along an axis X'X each comprising on the one hand a magnetic circuit (12; 22) defining with said body an air gap (ej; e2) whose average value is equal to a predetermined common value e ₀ , and d ' on the other hand an excitation coil (11; 21) supplied from a power amplifier (13; 23) whose input current is controlled as a function of the position of the body (1) relative to the magnetic circuits ( 12; 22) of the first and second electromagnets, characterized in that the power amplifiers (13, 23) are constituted by switching amplifiers; in that the coils (11; 21) of the first and second electromagnets directly constitute position detectors of the inductive type; in that means (14; 24) are provided for simultaneously injecting at the input power amplifiers (13; 23) supplying the coils (11; 21) of the first and second opposing electromagnets, superimposed on the main current from the servo circuits (31, 32), a sinusoidal current I ₀ sin wt of constant amplitude I ₀ , of pulsation w and having an identical phase, the ratio between the detection voltage supplied by an amplifier (13, 23 ) at the pulsation w and the voltage intended to supply the force exerted by the corresponding electromagnet being of the order of 5 to 20% and preferably close to 10%; and in that it comprises circuits (41 to 44) for extracting the position information determining the intensity of the main current to be applied by the control circuits (31, 32) to the power amplifiers (13, 23) directly from the voltages uj, U2 across the excitation coils (11; 21), measured at the frequency of the sinusoidal current constituting a pulsation carrier w.

2. Active magnetic bearing according to claim 1, characterized in that the frequency of the sinusoidal current injected with pulsation w is clearly greater than the desired bandwidth in closed loop of the servo circuits.

3. Active magnetic bearing according to claim 1 or claim 2, characterized in that the switching amplifiers (13, 23) have a switching frequency of the order of several tens of kilohertz.

4. Active magnetic bearing according to any one of claims 1 to 3, characterized in that the switching frequencies of the switching power amplifiers (13, 23) and the frequency of the sinusoidal pulsating current w are synchronized.

5. Active magnetic bearing according to any one of claims 1 to 4, characterized in that the voltages u, U2 at the terminals of the excitation coils (11, 21) measured at the frequency of the sinusoidal pulsating current w are compared by differential measurement in a comparator (41) providing a voltage u proportional to the movement of the body (1) relative to its equilibrium position.

6. Active magnetic bearing according to any one of claims 1 to 5, characterized in that the circuits (41 to 44) for extracting the position information comprise a bandpass filter (42) centered on the frequency of the sinusoidal pulsating current w and having a half-width adapted to the desired bandwidth of the control circuits (31, 32).

7. Active magnetic bearing according to claim 6, characterized in that the circuits (41 to 44) for extracting the position information comprise a synchronous demodulator (43) placed at the output of the band pass filter (42) and controlled by the frequency of the sinusoidal pulsating current w.

8. Magnetic bearing according to claim 7, characterized in that the circuits (41 to 44) for extracting the position information comprise a second order filter (44) for filtering the frequency corresponding to a pulsation 2w created by the full-wave demodulation within the synchronous demodulator (43).

9. Active magnetic bearing according to any one of claims 1 to 8, characterized in that the electromagnets are dimensioned in such a way that the maximum force required from these electromagnets is obtained for a magnetic induction situated outside the zone. saturation of the material used to form the magnetic circuits (12; 22) of the electromagnets.

10. Active magnetic bearing according to claim 9, characterized in that the magnetic circuits (12; 22) of the electromagnets are produced in a material with slightly variable permeability as a function of magnetic induction, in particular in the electromagnet operating zone located far from the saturation zones.

11. Active magnetic bearing according to any one of claims 1 to 10, characterized in that the control circuits (31, 32) supplying the power amplifiers (13; 23) comprise a comparator (31) between the signal of position from circuits (41 to 44) for extracting position information and a position reference value, and at least circuits (32) comprising a signal processing correction network and a linearizer of the quiescent current.

12. Active magnetic bearing according to claim 11, characterized in that the control circuits further comprise flow regulation loops interposed between on the one hand the circuits (32) comprising a corrector network and a linearizer, and on the other hand adder circuits (14, 24) constituting said injection means receiving said sinusoidal current I ₀ sin wt of pulsation w.

13. Active magnetic bearing according to claim 12, characterized in that each flow regulation loop comprises a rejector filter (15;

25) centered on a pulse frequency w and receiving as input the output voltage signal from the corresponding amplifier (13; 23), a flux calculation circuit (16; 26) to which the signal is applied on the one hand of the corresponding rejector filter (15; 25) and on the other hand the output signal of a flow regulator (18; 28) also applied to an input of the corresponding adder circuit (14; 24) and a subtractor circuit ( 17; 27) which subtracts from the signal from the circuits (32) comprising a correction network and a linearizer, the signal from the corresponding flux calculation circuit (16;

26) and delivers an output signal to the input of the corresponding flow regulator circuit (18; 28).

14. Active magnetic bearing comprising two active magnetic bearing systems according to any one of claims 1 to 13, oriented in two directions XX ', YY' perpendicular to each other.